Restructured directories for source code

[vincentlaucsb]

Restructured source code to make it cleaner and easier to navigate. Prereq to including third-party libraries (like string_view implementations)

[codecov]

Codecov Report

Merging #17 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #17   +/-   ##
=======================================
  Coverage   96.26%   96.26%           
=======================================
  Files           7        9    +2     
  Lines         563      563           
=======================================
  Hits          542      542           
  Misses         21       21

Impacted Files	Coverage Δ
include/internal/csv_stat.cpp	98.27% <ø> (ø)
include/internal/data_type.cpp	82.22% <ø> (ø)
include/internal/csv_row.hpp	93.75% <ø> (ø)
include/internal/csv_reader.cpp	96.25% <ø> (ø)
include/internal/csv_reader_iterator.cpp	95.83% <ø> (ø)
include/internal/csv_row.cpp	100% <ø> (ø)
include/internal/csv_utility.cpp	100% <100%> (ø)
include/internal/csv_format.hpp	100% <100%> (ø)
include/internal/csv_reader.hpp	100% <100%> (ø)
... and 2 more

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